Dissection of a Blu-ray Reader Assembly: The Laser Diode

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Read FIRST!

If you are new to diode lasers, read this article carefully before building or even disassembling the Blu-ray optics block! This will avoid costly mistakes! Carefully read all the related material in Sam’s Laser FAQ, especially in the chapters starting with: “Diode Lasers”. The information there is invaluable, and extremely important!

Precautions

All laser diodes, and especially the violet variety, are extremely ESD sensitive! So take serious antistatic precautions! Make sure ALL of your capacitors in your driver are FULLY discharged before connecting your diode to a driver. (Shorting just once is NOT enough, short them, and leave them shorted until your diode is soldered There should also be a resistor soldered permanently across the driver to do this automatically but don’t depend on it!!!!

Laser Diode Extraction

Trying to test the diode in-situ, resulted in spurious readings, presumably from the drive circuitry, so this is not recommended. There is a flat flex-cable from the diode to the main assembly. These diodes are easily damaged by ESD. Take extreme precautions when extracting and soldering! Solder to the GND pin first and solder the other end of the wire to the VLD anode (+ve), thus shorting the diode. This should make it safe for extraction and handling. When you are ready to connect the diode to your driver, you can then snip the shorting wire.

Once you have soldered on your shorting link, you may slice through the flex-cable with a sharp knife. Two small screws must then be removed on the little metal mount for the diode, and then it can be eased out (still attached to the mounting bracket/heatsink).

If you wish the diode can be removed from its mount, by carefully prying it off with a craft knife. The remaining glue will have to then be removed.

And here it is:

(Click on photo for closeup.)

This shows two views of the laser diode can along with the PCB that would normally attach via the flex-cable to the driver circuitry. Note the 5 legs on the laser diode can and five corresponding holes in the PCB. If the soldermask gets damaged on the ground plane on the PCB, there will very likely be solder shorts when you attempt to attach your ultra-fine wires! Carefully inspect your soldering before applying power!

Laser Diode Specifications

The laser diode can actually contains three laser diodes emitting at 780, 660, and 405 nm (for CD, DVD, and Blu-ray, respectively). These are connected in a CCC (Common Cathode Can) configuration. The case is the cathode (-ve) of all three laser diodes as well as the monitor photodiode.

The pinout for the PS3 diode is shown below (looking at the rear of the can).

Pinout:

VLD: Violet (405 nm) laser diode anode (+ve).

RLD: Red (660 nm) laser diode anode (+ve).

IRLD: Infra-red (780 nm) laser diode anode (+ve).

PD: Monitor Photodiode anode.

This is a close-up of the connections on my unit. (Note the remaining section of flex-cable PCB.)

Black: LD and PD cathodes (GND).

Blue: VLD anode (+ve).

Red: RLD anode (+ve).

Now typical violet laser diodes, according to whatever spec sheets you get your hands on, run from anywhere between 4.5 and 5.5 V, and 35 to 90 mA!

For Example Nichia diode NDHV310APB:

Item

Symbol

Min

Typ

Max

Unit

Optical output

Po

–

30

–

mW

Peak wavelength

λp

400

408

415

nm

Threshold current

Ith

–

45

60

mA

Operating current

Iop

–

70

90

mA

Slope Efficiency

η

0.7

1.2

1.6

W/A

Operating voltage

Vop

–

4.5

5.5

V

FWHM beam divergence
slow axis

θ

5

8

13

°

FWHM beam divergence
fast axis

θ

18

24

30

°

Monitor current

Im

0.1

0.25

0.6

mA

This is one of Nichia’s higher powered diodes (the one in the PlayStation-3 outputs 20 mW max). Note the variation in Ith of anywhere between 45 and 60 mA! And Iop of 70 to 90 mA! These specs, while not applicable to the PS3 diode, are important, as they give an idea as to the properties and variations in specification of Gallium Nitride laser diodes, i.e., once I work out Ith for my device, I can extrapolate Iop to within reasonable limits. So I read every 405 nm diode datasheet I could get my hands on!

A series of tests were conducted on the Violet laser diode and the following parameters were determined. (these are a guide only, and are the specs for my particular diode, your Ith may vary, and as a consequence, your operating current will vary!)

The above graph is a plot of diode current, against optical output, and was measured using a Coherent Lasercheck As can be seen on the graph, threshold (Ith) occurs at 28 mA to 30 mA.

Note: On this graph, the final real measurement was made at 45.7 mA (13.9 mW). The line from 45.7 mA to 50 mA is extrapolated.

Sam ran some tests of the infra-red, red, and violet laser diodes, and the monitor photo diode using a laboratory laser diode controller, the LDX LDC 3900. The results are summarised here:

Diode

Ith

Iop(5 mW)

Ipd (5 mW

Infra red ~780 nm

26 mA

34 mA

485 uA

Visible Red ~660 nm

22 mA

29 mA

355 uA

Violet ~405 nm

35 mA

49 mA

115 uA

Ith: Threshold current for the LD under test.

Iop (5 mW): Current required to produce 5 mW.

Ipd (5 mW): Current produced by the monitor photodiode at 5 mW.

Note the high Ith and Iop of the violet diode, as I stated, this had been ESD damaged, and thus had poor beam quality, and reduced output power. However, this at least gives us an indication of the response of the photodiode.

Determining Ith

There have been reports of diodes with thresholds ranging from 18 to 35 mA! The diodes are manufactured by a batch process, as opposed to continuously, and there MAY be significant variation in operating parameters. Always err on the side of caution and start your adjustments from 0 mA! It may well be that Ith may be lower or higher than expected! Once Ith has been determined for your particular diode, it is reasonable to conclude that the operating current range is Ith to Ith+10 mA, this should keep you within safe limits. Your diode voltage drop may also vary from ~4.4 V to 5.5 V.

To determine Ith, hook up the diode to your supply set at 0 mA (remember to have discharged you caps first!) Place a white card in front of the laser (the fluorescence will help determine the threshold). Gradually increase the current. Early on (possibly starting even at a fraction of 1 mA!!) there will be considerable violet emission, this is LED emission, and the device is not yet lasing, and will appear quite violet on the card. Once the diode reaches the lasing threshold, the light emitted will no longer be a diffuse circle but will take the form of a blue (because of fluorescence) bar, with various lines around the edges, and speckle should be noticeable. It will be quite dim at threshold, so keep a keen eye out.

Once you have determined Ith, make a note. Your maximum operating current (Iop) is around Ith+10 mA. So if Ith=28 mA, your maximum current ought to be 38 mA.

In the above picture the laser and its little mount were just glued to the large heatsink, no heat transfer compound was used. (The heatsink was used because it was convenient.) I figured there was enough metal in the mount to dissipate any heat. The laser stayed cool throughout its operation. In the above picture, a common acrylic asphere lens was used as the collimator. It was just glued to a PCB and focus was adjusted by moving the PCB either towards or away from the diode.

The diode can is standard 5.6 mm diameter, and could be easily integrated into the collimating assembly of a dead red module. A far superior method of mounting is to obtain a decent red laser pointer, and remove the collimator from it (the good ones are made of brass, and are adjustable). It is then a matter of simply installing the diode, where the red one was, and adjusting the focus to suit.

Output Measurements

The optical output was confirmed by Rob as being 10 to 20 mW. (If you really push it!, I would say 15 mW is an ultimate upper limit if the thing to last any length of time!) There will be more info about this, as soon as he completes his measurements.

The raw output beam, before collimation is very ugly, and appears very multimode:

The above image, was recorded by a digital camera and processed with ImajeJ. The satellite beams and other garbage can be quite easily seen.

As far as I can tell the diode is unsuitable for holography, although I have never tried it. The output is quite noisy, and appears multimode, but the use of laser diodes for holography has been proven before! It was once suggested that the coherence lengths of red laser diodes was sub-millimeter, but has been proven to surpass the coherence length of even the best gas lasers on the market! At some point I will measure the coherence length and post it here. I would imagine that as the technology progresses, we could reasonably expect to see single mode violet diodes on the market within 18 months.

Laser Diode Power Supply

A suitable power supply for the violet diode is shown below. Its nothing novel or exciting, but it works so long as your application isn’t too critical as far as optical noise or output variation is concerned.

It is a simple current regulator based on the LM317, and is easy to build, and can be made quite compact. There will shortly be a circuit diagram available with light feedback and the capability of running all three diodes, so watch this space!

The Stacked Diode Assembly

The laser diode can in the PS3 contains three laser diodes. The Red 660 nm diode is mounted next to the Infrared 780 nm diode, and both are bonded to the 405 nm Gallium Nitride violet diode. This is a very novel approach since it is exceedingly difficult to bond either AlGaAs or AlGaInP to the GaInN diode due to lattice mismatching. So far, the other Blu-ray pickups I have seen, have two objectives, and separate beam paths for the red/Ir and violet.

This particular design shows a four pin can, since the photodiode is not present. (Sony was only patenting the stacked structure).

Since I can’t really show the diagrams from the patent without copyright infringement, I have displayed some photographs of the diode structure below. These were kindly sent in to the site by John Rehwinkel (www.vitriol.com) whose violet diode can be seen in his Journal..

Here is a close-up of the front of the diode. You can see the multiple connections to the device.

Here is the diode with the red emitter powered.

And a close-up of the facets. The large, rhomboid structure sits on top of the Red and Infra-red diodes (which are very small!) The large slab on the bottom, is GaN. note the fracture on the left corner of the GaN slab.

This is a photo of the violet diode powered. Note the amount of light leaking from the left of the structure, where the afore mentioned fracture is. Although you should note that some light leaks out of all of the facets of the GaN die.

For More Information

There have been extensive discussions on the USENET newsgroup alt.lasers, and the Internet forums Photon Lexicon and Laser Pointer Forums. Checking the archives for those will probably turn up more than you ever wanted to know. You can contact Leslie at: lesliewright8@hotmail.com and Sam through the Feedback form at RepairFAQ.org. For a wealth of information about Laser related stuff, and plenty of diagrams, go to Sam’s Laser FAQ.

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